Measuring power with
a Fluke ScopeMeter®
190 Series
Application Note
Although many electrical test tools are available to measure
voltage, few can measure current, and even fewer are
equipped to measure electrical power directly. Moreover,
there is always the question of how to measure power in
electronic systems that are not operating at mains frequencies. The Fluke ScopeMeter 190 Series has the answer.
Power measurements
The prime parameter specified
for any electrical system is operating voltage. But that voltage in
itself has little meaning if, once
applied, it doesn’t result in
power to, for example, exert a
force, drive a machine or run a
lighting system. Accurate measurement of electrical power is
therefore crucial for understanding a system’s behaviour.
Suppose we have an electrical
circuit consisting of a voltage
source U1, a switch Sw and a
load resistor RL (see figure 1). By
closing the switch, the supply
voltage U1 is connected to the
load RL and a current I will flow
as a result. Once this current is
flowing, the load will heat as a
result of the power applied to it
according to:
P = U1 * I
(1)
This elementary power measurement, using a DC-source for
U1 and a resistive load RL
requires little more than a multimeter, which will usually be a
digital multimeter (DMM). The
DMM is used first to measure the
voltage U1 and then to measure
the current supplied to the load
RL, from which power supplied
to the load is calculated from formula (1).
Once the switch is closed, all
values in this DC-system are
static and therefore the two
measurements can be made successively using a single instrument if so desired.
If the source is a low-frequency AC source, the same
rules basically apply. We can
measure the RMS value of the
applied voltage and that of the
current, and multiply these to get
the power handled by the load.
As long as we’re working at
mains frequencies, most DMMs
can do the job.
If the load isn’t purely resistive but also includes inductive
or capacitive elements, a phase
difference between the applied
voltage and the current will
result that needs to be taken into
account to determine the power.
The relation now is:
P = U1 * I * cos ϕ
(2)
where j is the phase-angle in
degrees between voltage and
current.
Multimeters are generally not
equipped to make measurements
on more than one input and
therefore cannot measure phase
angle. For these measurements, a
more specialized instrument is
needed, for instance a phase
meter or an oscilloscope.
Figure 1. Basic circuit loop in which voltage
and current are measured.
From the Fluke Digital Library @ www.fluke.com/library
If power measurements are
made more frequently, a power
meter like the Fluke 43B Power
Quality Analyzer would be the
most appropriate test tool. This
instrument measures both voltage and current simultaneously
and automatically takes the
phase angle into account. For the
occasional user, however, this
may be an expensive option and
a more generic tool like the
ScopeMeter would be preferred.
Use of the Fluke
ScopeMeter
All Fluke ScopeMeters have two
input channels and can measure
voltage and current at the same
time, whilst also measuring and
displaying the phase angle
between them. See also the section “practical set-up hints” near
the end of this publication for
further details.
If the voltage is non-sinusoidal or if the load isn’t purely
resistive, power measurements
become too complex to be performed on a DMM. The best way
to determine power under these
conditions is to take a large
number of current and voltage
measurements over each cycle of
the supply voltage. The measurements need to be made simultaneously on both signals. Each set
of simultaneous measurements
can then be multiplied to produce a corresponding set of data
points from which a curve can be
constructed of the power handling at successive moments in
time.
The Fluke 190 Series
ScopeMeter is capable of performing this specific function for
you!
Included in the functionality of
the Fluke ScopeMeter 190 Series
is the ability to multiply individual curves (waveforms) to create
a resultant curve. With this function, sets of samples from channels A and B are multiplied to
create a resultant curve labeled
M. In other words, every time a
sample for channel A and a sample for channel B is taken and
written on screen, these are also
multiplied and displayed to create the resultant waveform M on
screen. From this waveform, the
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(instantaneous) power can be
read off, at every point in time,
for instance using the cursors of
the Fluke 190 Series.
Example of a light
dimmer
As an example, we’ve applied
the mains voltage to a so-called
“light dimmer”. This is a thyristor-based power control device
in which current is allowed to
flow only during a selected part
of the mains cycle. The effective
output voltage to the load (the
lights) can be controlled by
changing the phase angle. The
measured voltage (see figure 2,
trace A) shows that the output is
active during approximately twothirds of the time, or about 120
degrees during each half-cycle.
During one-third of the time, the
dimmer is ‘off’ and no output
voltage is supplied to the lights.
If we change this phase angle,
the lights will brighten or dim.
Figure 3. Menu tree for setting up the waveform multiplication.
scaling can be modified. After
pressing softkey F3 we can also
change the vertical position of
the resultant trace M.
Figure 4. Voltage, Current and Power curves
on the Fluke 190 Series ScopeMeter.
Figure 2. Output voltage and current of a
dimmer driving a string of lights.
In figure 2 we can also see
the resulting current through the
light-bulbs (channel B, blue
curve). We can now set up the
ScopeMeter to calculate the
power applied to the lights (see
figures 3 and 4).
To do this select:
SCOPE,
then F4 (= Waveform Options),
next Mathematics, and Enter,
then A * B, and Enter.
Next we select a scaling that
we expect will keep the power
curve on screen; this may also be
changed later. The two input
waveforms and the resultant (M)
will now be visible. If the resultant is too small or too large, the
Measuring power with a Fluke ScopeMeter® 190 Series
Figure 4 shows the voltage
(waveform A, in red), the current
(B, in blue) and the multiplied
curve (M, in green), representing
the power supplied to the lights.
On the Fluke ScopeMeter 190
Series, the cursors can be used to
measure the power at any point
in time, as shown in figure 5.
Figure 5. The ScopeMeter’s cursor is used for
power measurement at a specific point in time.
Here, the cursor is set at about
the maximum peak of the power
curve, and the reading tells us
that the lights are handling a
peak of approximately 1.7 kW at
that instant in time.
Power measurement in a
switched-mode power
supply (SMPS)
In electronic systems, the frequencies of the signals are often
much higher and the waveforms
much more diverse than those in
above example.
As an example, consider some
waveforms from a switchedmode power supply (figure 6). In
this system, the mains voltage is
rectified and filtered, resulting in
a DC voltage of about 350 V. This
is then applied to a switching
transistor that drives a stepdown transformer.
Figure 7. Voltage and current waveforms are
used to create the power-curve.
From the curves in figure 6,
we calculate the power that the
transistor is handling by multiplying the two graphs, as in figure 7. Here also the
timebase-setting has been
changed to see the part of the
waveform that is of particular
interest in more detail.
On these curves, we can use
the cursors of the Fluke 190
Series to measure the peak in
power handling, as is done in
figure 8.
Figure 6. Voltage and current handled by the
switcher component in a SMPS.
In figure 6 we can see the
voltage across the switching
transistor (curve A, in red) and
the current through the transistor
(curve B, in blue). The voltage
reaches peak values of over
400 V (see the 4 divisions of
amplitude at 100 V/div) and the
current has a peak-value of over
200 mA. A single cycle of this
converter’s signal takes approximately 26 µs, which means the
operating frequency is around
36 kHz. Note, however, that for a
given SMPS, the frequency may
vary with changes in the line
voltage and loading.
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Figure 8. Cursor used to measure the peak of
the power, handled by the transistor.
From this we see that the
switching transistor is handling a
peak power of 123 W. During
design of the SMPS, one has to
be aware that such power peaks
may occur and the components
need to be selected with this in
mind.
Instrument set-up
The ScopeMeter is unique in that
its inputs are fully insulated from
one another, allowing direct connection to electrical wiring, even
if this is at mains potential. Most
standard oscilloscopes have a
common ‘ground’ connection on
the inputs which has to be used
as a common reference point for
Measuring power with a Fluke ScopeMeter® 190 Series
all input signals (and which is
connected to the mains’ safety
ground). In contrast, the
ScopeMeter 190 Series has independently floating inputs. This
allows for voltage measurements
at voltage test-points as well as
across current-sensing resistors,
which may be at different voltage levels. See the ScopeMeter’s
technical specification for full
details.
When used with current
clamps or with current sensing
resistors in the network, the
ScopeMeter inputs can be set-up
to read amplitudes in amps
directly.
To do so, select the input
channel key (e.g.: “B”) and then
softkey F3 (“Probe channel B”). A
selection can then be made if the
input signal represents a voltage
or a current, or even a temperature. What’s more, the sensitivity
of the current clamp or the circuit’s sensing resistor can be
selected from a selection table,
indicated in mV/mA (equivalent
to V/A or simply Ω).
If we have connected a 1 Ω
current sensing resistor in our
network and want to measure
the voltage across it, the current
sensitivity would be set at 1 V/A.
If we include a 0.1 Ω resistor in
our network to sense the current,
then the sensitivity would be set
at 100 mV/A. If the voltage
across that resistor is measured
using the standard (10:1) voltage
probe, the overall sensitivity
would be 10 mV/A. By selecting
this sensitivity from the menu,
the current that we read on the
ScopeMeter screen directly indicates the true value.
Total energy
The total energy handled over
time can be calculated by multiplying the continuous power and
the time the system is active:
W=P*t
(3)
The result is expressed in
watt-seconds (Ws), also known
as joules (J). The value in joules
can be rescaled into kWh,
whereby
1 kWh = 1000 Wh =
1000 * 3600 Ws = 3.6 * 106 Ws
Some ScopeMeter models also
contain a function for calculating
the total power accumulated over
a period of time, which is then
selected using the cursors. Power
over time equals energy, and we
can read this on the ScopeMeter
as watt-seconds directly.
Figure 9. Power curve and energy
measurement.
Looking at figure 9, we can
see that between the two cursors, i.e. within a single cycle of
the mains voltage, an energy of
10.09 Ws is delivered to the
lights. A cycle of the mains in
this case takes 20 ms.
Per second this leads to a
power consumption of:
10.09 Ws / 20ms =
10.09 * 50 W*s/s = 505 W
And over a time-span of one
hour, this equals a total energy
consumption of:
505 W * 1 h = 505 Wh =
0.505 kWh = 1.8 MWs
Practical set-up hints
The best way to measure a current in a circuit loop is with a
current clamp. These are commercially available for AC and for
DC+AC measurements, and for
various current ranges. The Fluke
80i-100s, for instance, measures
DC and AC currents from 0.1 to
100A; the Fluke i1010 can even
be used up to1000A AC and/or
DC.
These clamps eliminate the
need to open up the circuit loop
when making measurements and
provide good isolation between
any ‘live’ wiring and the test
instrument. When working on
power circuits, this is definitely
the safest way to measure the
current.
If a current clamp is used for
measuring small currents and the
sensitivity of the clamp in insufficient, the effective sensitivity can
be increased by feeding multiple
turns of the wire through the
clamp. The actual current is now
the measured current divided by
the number of turns.
Sometimes, however, it’s not
so simple to cut the wiring of an
existing circuit loop in order to
include a current meter, for
example when all wiring is part
of a printed circuit board. A possible way to bring the current
meter into the loop of a lowpower circuit would then be to
set up the DMM for current
measurement and select the
highest current-range provided.
Now connect the meter over the
contacts of the on/off-switch. If
the switch is left open, the meter
will close the loop and read the
current, while no wiring needs to
be interrupted or modified.
If no switch is provided, or if
the DMM has no current-measuring capability, we may also add
a current sensing resistor of a
known value Rs to the circuit,
which needs to be small in value
compared with the load-resistance RL (see figure 10). We can
now measure the voltage across
this sensing resistor and calculate the current from Ohms’ law.
Adding the resistor is a one-time
modification, which is more convenient than repeatedly opening
up the circuit loop.
If Rs is more than 10 times
smaller in value than the load
RL, less than a percent of the
energy will be handled by the
series resistance, and thus the
error in the power measurement
that results from adding the
resistor will be less than 1%.
Conclusion
Power measurements on low-frequency linear systems can be
performed using a DMM. The
measurement of power in electronic systems where waveforms
are more complex and frequencies often much higher than the
mains frequency requires more
sophisticated tools. The Fluke
190 Series of ScopeMeters are
well equipped to make these
measurements, and can even
make fast peak-power measurements to determine the power
handling of fast electronic components, for instance in
switched-mode power supplies.
Fluke. Keeping your world
up and running.
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
Figure 10. Adding a current sensing resistor to
the circuitry to allow current measurement.
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
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Fax (31 40) 2 675 222
In Canada (800) 36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.fluke.com
©2004 Fluke Corporation. All rights reserved.
Printed in U.S.A. 3/2004 2140127 A-ENG-N Rev A
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Measuring power with a Fluke ScopeMeter 190 Series
®